Ibix said:
We don't know what dark matter is. We can see that galaxies are (nearly) all spinning fast enough that they'd fly apart if the only thing holding them together were the gravity of the matter we can see, so there must be something we can't see. The leading idea is that there's some kind of matter out there that doesn't interact with light (hence "dark matter", although "transparent matter" might have been a better name), perhaps some new particle or a population of small black holes or something. However, attempts at direct detection of dark matter of any type people have thought of have so far come up empty.
A significant minority position is that actually our understanding of gravity is wrong. Modified theories of gravity have been proposed and have had some success at explaining galaxy dynamics, but they are incompatible with general relativity so have trouble explaining observations in other fields that do require general relativity.
Finally, there is at least one group that says that dark matter is actually just an effect of general relativity, and that if you apply general relativity correctly (or what they say is correctly) then galaxies behave exactly as our current theories predict and no new theory or dark matter is needed.
In short, there are a lot of different ideas and we don't know which - if any - is correct.
I would add just one point to clarify this solid answer.
The astronomy phenomena attributed to dark matter are more or less universally agreed to happen and have been documented in great detail.
There is not really any doubt about the validity of the myriad examples of observational evidence that show that general relativity, as conventionally implemented and applied, cannot explain what astronomers see with ordinary matter alone.
The debate among astrophysicists is over the cause of these phenomena. This has proven to be a difficult question to solve. Every approach used to explain these phenomena has serious problems at explaining all of the data related to dark matter phenomena in a way that can gain wide acceptance, even though each approach can explain some dark matter phenomena observations.
And, each of the three main approaches outlined has multiple different theories within it. There are probably at least a dozen often very different serious proposals for the nature of dark matter particles. There are several different leading modified gravity proposals. And, there are at least a three different ways of applying orthodox general relativity in an unconventional manner that have been proposed to explain what astronomers observe.
There are a few instances where a particular observation (such GAIA space telescope observations suggesting possible dark matter/modified gravity effects in wide binary star systems) are disputed due to possible systemic error in astronomy observations, and due to tricky statistical analysis issues for the large data sets involved (often involving many thousands of astronomy observations of particular stars and galaxies). The observations where there are uncertainties and disputes sometimes favor one mechanism to explain dark matter phenomena observed over others, depending upon how they are resolved, which makes the debate over the correct interpretation of these observations rather heated.
But these disagreements are the exceptions rather than the rule, and involve observations that are trying to pin down the fine details about exactly how dark matter phenomena work. There are not disagreements over the existence of some kind of dark matter phenomena at all, which has been well established since they were first definitively documented by
Vera Rubin and other astronomers in the 1970s. These phenomena have continued to be confirmed right up through the observations of the James Webb Space Telescope which is the latest and best telescope ever built (at least for some purposes).
The interdisciplinary scientific effort to better understand the unsolved problem of dark matter phenomena is the subject of many, many experiments and theoretical investigations from every imaginable perspective. Many new papers are published every week tackling little corners of the problem with new observations and analysis. Arguably, it is the single most important unsolved problem in fundamental physics.
So, while we don't know what causes dark matter phenomena now, we are making progress in ruling out many possible explanations, and we are narrowing down what properties any successful theory to explain these phenomena must have. We probably won't have a definitive answer a decade from now. But we are making progress and will probably someday have a much better answer.
